1. Field of the Invention
The present invention relates to in-line shift transmissions. More specifically, the field of the invention is that of shift keys for in-line shift transmissions.
2. Prior Art
In-line shift transmissions include a plurality of linked transmission shafts, with one of the shafts being a shift shaft supporting a plurality of rotatable gears. Disposed axially within the shift shaft are keys which can engage internal keyways of the gears and thereby rotatably connect the engaged gear to the output shaft. The key comprises a resilient metal having an elongate shaft and a head portion which is biased to extend into the gear keyway.
An in-line transmission is disclosed in U.S. Pat. No. 4,996,574, assigned to assignee of the present invention, the disclosure of which is explicitly incorporated by reference. The shifting mechanism of the '574 transmission includes a pair of axially movable shift keys disposed in longitudinal channels of the shift shaft. The shift keys rotatably couple the shift shaft with one of the gears. Each key has a base portion engaging a shift collar which is axially slidable on the shift shaft. The collar has a peripheral groove receiving a pin of a shift fork. The shift fork can pivot within the housing and is connected to the shifting mechanism of the vehicle.
The shift fork, pin, and annular groove permit the collar, and hence the keys, to be moved axially while the keys are at the same time rotating with the shift shaft. Each of the gears journalled on the shift shaft has on its inner circumference four circumferentially spaced keyways which may be engaged by the keys to rotatably couple the shift shaft with the selected gear. The keys have triangular key heads and flexible spring shank portions which spring bias the key heads radially outward and into the keyways. Most of the key shaft is rotatably supported by the channel of the shift shaft, and most of the key head rotatably engages the sides of the gear slot. Thus, while the keys are radially resilient they are also rotatably rigid.
Between each adjacent gear on the shift shaft are annular washers which separate adjacent keyways and which, acting on the triangular sides of the key heads, force the heads out of each keyway upon axial movement of the collar and keys. Consequently, as the keys are moved axially by the shift collar, the key heads successively engage and disengage the keyways of adjacent gears, thereby coupling each gear to the shift shaft sequentially one at a time.
One problem encountered by in-line shift transmissions involves shearing forces on the keys when they are engaged with a keyway. Most of the key head extends into the keyway so that the connecting portion between the triangular key head and the key shaft receives all the twisting and shearing force of the load. When high shearing forces are applied to a shift key, a connection failure between the key head and keyway may occur, or the key may break. Some prior art transmissions limit shearing forces on the keys by increasing the diameter of the shift shaft, because the amount of shearing force which acts on the connecting portion of the keys decreases as the diameter of the shift shaft increases. However, the increased diameter of the shift shaft limits the possible range of gear ratios. Therefore, providing a large range of gear ratios and a smaller transmission size runs against known methods of reducing stress on the shift keys.
What is needed is a transmission shift key arrangement which provides a compact transmission having a large range of gear ratios.
Also needed is a transmission which reduces the stress applied to the shift keys.
Additionally needed is a shift key which can better withstand shearing forces from the shift shaft.
A further need is for an improved shift key which is compatible with existing transmission designs.